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open(3perl)	       Perl Programmers	Reference Guide		   open(3perl)

NAME
       open - perl pragma to set default PerlIO	layers for input and output

SYNOPSIS
	   use open IN	=> ":crlf", OUT	=> ":bytes";
	   use open OUT	=> ':utf8';
	   use open IO	=> ":encoding(iso-8859-7)";

	   use open IO	=> ':locale';

	   use open ':encoding(utf8)';
	   use open ':locale';
	   use open ':encoding(iso-8859-7)';

	   use open ':std';

DESCRIPTION
       Full-fledged support for	I/O layers is now implemented provided Perl is
       configured to use PerlIO	as its IO system (which	is now the default).

       The "open" pragma serves	as one of the interfaces to declare default
       "layers"	(also known as "disciplines") for all I/O. Any two-argument
       open(), readpipe() (aka qx//) and similar operators found within	the
       lexical scope of	this pragma will use the declared defaults.  Even
       three-argument opens may	be affected by this pragma when	they don't
       specify IO layers in MODE.

       With the	"IN" subpragma you can declare the default layers of input
       streams,	and with the "OUT" subpragma you can declare the default
       layers of output	streams.  With the "IO"	 subpragma you can control
       both input and output streams simultaneously.

       If you have a legacy encoding, you can use the ":encoding(...)" tag.

       If you want to set your encoding	layers based on	your locale
       environment variables, you can use the ":locale"	tag.  For example:

	   $ENV{LANG} =	'ru_RU.KOI8-R';
	   # the :locale will probe the	locale environment variables like LANG
	   use open OUT	=> ':locale';
	   open(O, ">koi8");
	   print O chr(0x430); # Unicode CYRILLIC SMALL	LETTER A = KOI8-R 0xc1
	   close O;
	   open(I, "<koi8");
	   printf "%#x\n", ord(<I>), "\n"; # this should print 0xc1
	   close I;

       These are equivalent

	   use open ':encoding(utf8)';
	   use open IO => ':encoding(utf8)';

       as are these

	   use open ':locale';
	   use open IO => ':locale';

       and these

	   use open ':encoding(iso-8859-7)';
	   use open IO => ':encoding(iso-8859-7)';

       The matching of encoding	names is loose:	case does not matter, and many
       encodings have several aliases.	See Encode::Supported for details and
       the list	of supported locales.

       When open() is given an explicit	list of	layers (with the three-arg
       syntax),	they override the list declared	using this pragma.  open() can
       also be given a single colon (:)	for a layer name, to override this
       pragma and use the default (":raw" on Unix, ":crlf" on Windows).

       The ":std" subpragma on its own has no effect, but if combined with the
       ":utf8" or ":encoding" subpragmas, it converts the standard filehandles
       (STDIN, STDOUT, STDERR) to comply with encoding selected	for
       input/output handles.  For example, if both input and out are chosen to
       be ":encoding(utf8)", a ":std" will mean	that STDIN, STDOUT, and	STDERR
       are also	in ":encoding(utf8)".  On the other hand, if only output is
       chosen to be in ":encoding(koi8r)", a ":std" will cause only the	STDOUT
       and STDERR to be	in "koi8r".  The ":locale" subpragma implicitly	turns
       on ":std".

       The logic of ":locale" is described in full in encoding,	but in short
       it is first trying nl_langinfo(CODESET) and then	guessing from the
       LC_ALL and LANG locale environment variables.

       Directory handles may also support PerlIO layers	in the future.

NONPERLIO FUNCTIONALITY
       If Perl is not built to use PerlIO as its IO system then	only the two
       pseudo-layers ":bytes" and ":crlf" are available.

       The ":bytes" layer corresponds to "binary mode" and the ":crlf" layer
       corresponds to "text mode" on platforms that distinguish	between	the
       two modes when opening files (which is many DOS-like platforms,
       including Windows).  These two layers are no-ops	on platforms where
       binmode() is a no-op, but perform their functions everywhere if PerlIO
       is enabled.

IMPLEMENTATION DETAILS
       There is	a class	method in "PerlIO::Layer" "find" which is implemented
       as XS code.  It is called by "import" to	validate the layers:

	  PerlIO::Layer::->find("perlio")

       The return value	(if defined) is	a Perl object, of class
       "PerlIO::Layer" which is	created	by the C code in perlio.c.  As yet
       there is	nothing	useful you can do with the object at the perl level.

SEE ALSO
       "binmode" in perlfunc, "open" in	perlfunc, perlunicode, PerlIO,
       encoding

perl v5.20.2			  2014-12-27			   open(3perl)

NAME | SYNOPSIS | DESCRIPTION | NONPERLIO FUNCTIONALITY | IMPLEMENTATION DETAILS | SEE ALSO

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